A team of federal safety experts investigating the CTA Blue Line train crash has focused on the operator’s physical condition and whether she was fatigued when the train struck a barrier and went airborne, an official said Tuesday.

Questions have been raised about whether the operator was sufficiently rested when her shift began around 8 p.m. Sunday, and about her level of awareness more than six hours later when the train plowed into the station and landed atop an escalator at O’Hare International Airport.

“We always take into consideration the fatigue factor,” said Ted Turpin, the lead investigator for the National Transportation Safety Board. “That's one of the areas we investigate.”

The effects of fatigue on train operators has been a well-documented factor in train accidents. The Federal Railroad Administration has estimated that “human factors“ contribute to nearly 40 percent of train crashes and that fatigue played a role in roughly a quarter of them.

The Dec. 1, 2013 Metro-North commuter railroad derailment in New York City in which four people were killed has drawn nationwide attention and its own federal investigation. The lawyer and union representing the engineer have reportedly said he admitted to “nodding off.”

In Monday’s crash, the train operator, a one-year CTA employee, was “extremely tired,” and there were indications she might have dozed off at the controls, according to the transit workers union president.

The NTSB provided no comment on a scheduled Tuesday-afternoon interview between the operator and investigators. The CTA has not identified her, nor would the agency answer questions about her shift and hours.

Union President Robert Kelly has said the operator “works a lot of overtime,” but had been off for about 17 hours before starting her overnight shift.

Experts have acknowledged the potentially dangerous combination of late shifts and sleep-deprived operators.

“The human machine...is not designed to work around the clock,” said William Sirois, chief operating officer of Circadian Technologies Inc., a research and consulting firm in Stoneham, Mass., specializing in shift work and extended-hours operations.

Major physiological changes occur at night. Blood pressure and body temperatures drop. Hormonal secretions change. Such biological traits increase risk, Sirois said, estimating the chances of having an accident or incident between 3 a.m. and 6 a.m. increases up to fifteen-fold.

Fatigue, according to a 2013 railroad administration report, is viewed as largely a function of sleep and human circadian rhythms – the brain-powered process that helps dictate human sleepiness and alertness on a roughly 24-hour cycle.

But fatigue exposure is determined largely by work schedules and contributes to the probability of “human factor accidents,” the federal railroad report said. The risk of human factor accidents are elevated by 11 to 65 percent above chance by fatigue exposure, the report said.

Brain signals that dictate human alertness levels peak during daylight hours for regular shift workers, said Dr. Phyllis Zee, associate director of the Center for Sleep & Circadian Biology at Northwestern University. Those signals tend to reach their lowest point around 3 a.m. to 5 a.m., Zee said, though humans can adjust their rhythm’s timing with practice.

Certain shifts in federal regulation and private industry practice have begun to address fatigue management. The NTSB, for example, has issued a host of safety recommendations intended to address fatigue-related incidents in the country’s skies, roads and railways.

Overall, the NTSB says employers should establish science-based management systems to identify factors that cause fatigue and detect its presence before problems occur.

In 2006, the NTSB recommended the Federal Transit Administration require transit agencies to ensure train operators’ time off between daily shifts — including regular and overtime assignments — allowed for at least eight hours of uninterrupted sleep.

Mass transit agencies need to “continually improve“ their understanding of the role of human error in accidents, the agency said in a 2014 publication. That knowledge, it said, should be used to strengthen practices, including fatigue management systems.

Despite such policy concerns, poor rest still contributes to accidents. “It’s happening, but unfortunately we’ve still got tired people at the controls,” Sirois said.

Meanwhile, other investigators at O’Hare focused on additional areas Tuesday, including the condition and maintenance of the tracks; the train cars and their mechanical condition; signals and switches; and other CTA personnel, Turpin said.

The investigators determined that the train’s emergency braking system was engaged, but the train apparently wasn't speeding as it approached the station, Turpin said. As the train approached the O’Hare station at about 2:50 a.m., it was traveling about 25 or 26 mph on the middle of three tracks, he said.

A safety device on the tracks triggered the train’s emergency braking system, but it wasn’t clear yet why the train did not stop in time, Turpin said.

“Whether it stopped in time or not, that’s an analysis we have to figure out,” he said.

The eight-car train plowed through a bumping post at the track’s end and came to rest atop an escalator. Were it not for the overnight hour, the platform and escalator would likely have been filled with people at the busy station. More than 30 people were hurt in the Hollywood-worthy crash though none of the injuries was considered life-threatening.

Investigators have recovered “event-type” recorders from the train, Turpin said. “They are...not like a black box (on an airplane), but they do have some minimal recording devices on the cars,” he said.

The NTSB also recovered “quite a bit of video evidence,” Turpin said. There are as many as 41 video cameras at the station, as well as the on-train videos, he noted. Cameras are on each car, and there is also a forward-facing camera on the operator’s car.

The video evidence is being forwarded to the NTSB in Washington, D.C., for analysis.

The lead train car will need to be “cut apart” to remove it from the escalator, Turpin said, adding that officials did not know how long it would take.

It also wasn't clear when train service to the airport would resume. Officials said they would continue shuttling passengers via buses from the O’Hare station to Rosemont.